How does the penetrating effect of different subshells present in a shell of an atom affect the ionization energy?
so to answer this firstly we have to understand ionisation energy.
It is the energy required in kj/mol to remove an electron from the outermost shell of an isolated gaseous atom.
now the question arises of penetration , it is the property of approaching near to the nucleus.
The deeply penetrated electron means that it is near to the nucleus .
So if an electron is deeply penetrated , it means that we have to spend more energy to remove it . and thus higher will be its ionization energy.
Here are the subshells in increasing order of their penetration power
f < d < p < s
The Unconditional Guru 